Catheters are often used in medical procedures to provide physical access to remote locations within a patient via a relatively small passageway, reducing the need for traditional invasive surgery. The catheter tube can be inserted into an artery or other passageway through a relatively small incision in the patient's body, and threaded through the patient's system of blood vessels to reach the desired target.
Various types of catheters are used in various procedures, both diagnostic and therapeutic. One general type of catheter used for both diagnostic and therapeutic applications is a cardiac electrode catheter. The diagnostic uses for a cardiac electrode catheter include recording and mapping of the electrical signals generated in the course of normal (or abnormal) heart function. Therapeutic applications include pacing, or generating and placing the appropriate electrical signals to stimulate the patient's heart to beat in a specified manner, and ablation. In an ablation procedure, electrical or radio-frequency energy is applied through an electrode catheter to form lesions in a desired portion of the patient's heart, for example the right atrium. When properly made, such lesions will alter the conductive characteristics of portions of the patient's heart, thereby controlling the symptoms of supra-ventricular tachycardia, ventricular tachycardia, atrial flutter, atrial fibrillation, and other arrhythmias.
Such a catheter is typically placed within a desired portion of the patient's heart or arterial system by making a small incision in the patient's body at a location where a suitable artery is relatively close to the patient's skin. The catheter is inserted through the incision into the artery and manipulated into position by threading it through a sequence of arteries, which may include branches, turns, and other obstructions.
Once the cardiac electrode catheter has been maneuvered into the region of interest, the electrodes at the distal end of the catheter are placed against the anatomical feature or area sought to be diagnosed or treated. This can be a difficult procedure. The electrophysiologist manipulating the catheter typically can only do so by operating a system of controls at the proximal end of the catheter shaft. The catheter can be advanced and withdrawn longitudinally by pushing and pulling on the catheter shaft, and can be rotated about its axis by rotating a control at the proximal end. Both of these operations are rendered even more difficult by the likelihood that the catheter must be threaded through an extremely tortuous path to reach the target area. Finally, once the tip of the catheter has reached the target area, the electrodes at the distal end of the catheter are placed in proximity to the anatomical feature, and diagnosis or treatment can begin.
In the past, the difficulties experienced by electrophysiologists in the use of a cardiac electrode catheter have been addressed in a number of different ways.
To facilitate maneuvering a catheter through a tight and sinuous sequence of arterial passageways, catheters having a pre-shaped curve at their distal end have been developed. To negotiate the twists and branches common in a patient's arterial system, the catheter typically is rotatable to orient the pre-shaped curve in a desired direction. Although the tip of the catheter may be somewhat flexible, the curve is fixed into the catheter at the time of manufacture. The radius and extent of the curvature generally cannot be altered. Therefore, extensive pre-surgical planning is frequently necessary to determine what curvature of catheter is necessary. If the predicted curvature turns out to be incorrect, the entire catheter may need to be removed and replaced with one having the proper curvature. This is an expensive and time-consuming ordeal, as catheters are generally designed to be used only once and discarded. Moreover, the additional delay may place the patient at some additional risk.
A variation of the pre-shaped catheter uses a deflectable curve structure in the tip. This type of catheter has a tip that is ordinarily substantially straight, but is deflectable to assume a curved configuration upon application of force to the tip. However, the tip deflection is not remotely controllable. In a certain patient's arterial system, a point may be reached at which the proper force cannot be applied to the catheter tip. In such cases, the catheter must be withdrawn and reinserted through a more appropriate passage, or another catheter with a different tip configuration must be used.
Another attempt to facilitate the placement of catheters takes the form of a unidirectional steering catheter. A typical unidirectional steering catheter has a steering mechanism, such as a wire, that extends the length of the catheter to the distal tip. The steering mechanism is coupled to the tip in such a way that manipulation of the proximal end of the mechanism (e.g., by pulling the steering wire) results in deflection of the catheter tip in a single direction. This type of catheter is illustrated, for example, in U.S. Pat. No. 5,125,896 issued to Hojeibane. The direction of deflection can be controlled by embedding a ribbon of wire in the tip; the ribbon is flexible along one dimension but not in others. This type of catheter can further be controlled by rotating the entire shaft of the catheter; in this manner, the direction of bend within the patient can be controlled. The shaft of such a catheter must be strong enough to transmit torque for the latter form of control to be possible.
Bidirectional steering catheters also exist. The distal end of a bidirectional steering catheter can be maneuvered in two planes, allowing the tip to be positioned with greater accuracy. However, bidirectional steering catheters are complex mechanically and are often difficult to manipulate.
Although the foregoing types of catheters address the issue of maneuverability in different ways, none of them is ideally configured to maintain contact with and apply a desired amount of pressure to a desired anatomical feature, such as an atrial wall.
One device used for the latter purpose is known as a basket catheter. See, for example, the HIGH DENSITY MAPPING BASKET CATHETER manufactured by Cardiac Pathways Corporation. A basket catheter has several spring-biased arms near the distal tip. When these arms are unconstrained, they bow outward to define a basket-like shape. The arms of the basket are constrained for implantation in a sheath structure. When the tip of the catheter has reached the desired location, the sheath is retracted, or the arms are advanced out of the sheath.
However, because the tip of the catheter is sheathed, it is not easily steerable into location, and is not as flexible as one might desire. Moreover, the sheath adds bulk to the device, which might significantly limit the range of applications in which the basket catheter can be used. The basket has only one shape and size. Once the arms are deployed from the sheath, the basket assumes a single configuration defined upon manufacture. If the predefined configuration of the basket is not suitable, then substantially no correction is possible. Also, known basket catheters are not indicated for use in high-energy therapeutic applications, such as ablation.
A variable-geometry sheathed electrode catheter is also known in the art. This device has a single electrode-bearing tip portion that is initially disposed within a relatively inflexible sheath. When the tip portion is advanced with respect to the sheath, the tip portion bows out of a slot-shaped aperture in the sheath. The shape of the tip portion can be controlled to apply a desired amount of pressure to an anatomical feature. However, as a sheath is used around the catheter, the device is not easily steerable into location. Moreover, as discussed above, the sheath structure adds undesirable bulk to the device.
Radio frequency ablation (RFA) has become the treatment of choice for specific rhythm disturbances. To eliminate the precise location in the heart from which an arrhythmia originates, high frequency radio waves are generated onto the target tissue, whereby heat induced in the tissue burns the tissue to eliminate the source of arrhythmia.
U.S. Pat. No. 5,617,854 to Munsif describes, inter alia, a pre-shaped catheter particularly useful for ablating in the vicinity of the sinoatrial node, the left atrium, and up to the mitral valve. The tip of the catheter is formed of a temperature-sensitive shape-memory material, e.g., Nitinol, or is otherwise invoked to assume a segmented configuration upon reaching a desired position. The segmented configuration includes proximal and distal segments which are generally parallel. The distal segment includes an ablation electrode. In operation, the segmented shape produces tension which urges the ablation electrode on the distal segment into contact with a wall of the left atrium, while the proximal segment is urged against other tissue. Since the shape of the catheter tip is fixed, the catheter tip is not easily manipulated. Further, the tension produced between the segments of the catheter tip is dependent on the shape and dimensions of the ablation site, e.g., the left atrium.
It is well known that aberrant heart activity such as arrhythmia may result from signals originating at the pulmonary veins. Unfortunately, it is particularly difficult to perform electrophysiological investigation and treatment at the pulmonary veins using existing ablation catheters.
Guiding and maneuvering a catheter towards and within the pulmonary veins is difficult due to the location, dimensions and structure of the pulmonary veins. Specifically, the average diameter of the pulmonary veins is on the order of 25 mm, i.e., at least one order of magnitude larger than the diameter of a typical catheter shaft which is adapted to be guided in relatively narrow arteries. This difference in dimensions makes it difficult to maintain continuous, controlled, contact between the catheter tip and the wall of the pulmonary vein. Further, it is difficult to manipulate the catheter tip from the relatively large space of the left atrium into a given pulmonary vein and to insert the catheter tip into the pulmonary vein. This limited maneuverability of the catheter towards and within the pulmonary veins is time consuming and results in inaccurate positioning of mapping and/or ablation electrodes along the inner walls of the pulmonary veins.
Further, the pulmonary veins have a rubbery tissue which is particularly susceptible to perforation and must, thus, be treated with extreme care to avoid damage. Unfortunately, the more maneuverable catheters described above, e.g., the "basket" type catheter and the variable-geometry, sheathed catheter, which theoretically could be used to engage the inner wall of the pulmonary veins, have complex structures which are most likely to damage the tissue of the pulmonary veins. Therefore, existing catheters cannot be used to safely and effectively investigate and/or ablate tissue along the inner walls of the pulmonary veins.
Accordingly, there is a need for a cardiac electrode catheter that can be conveniently steered into a relatively wide blood vessel, e.g., a pulmonary vein, and that can be controlled to efficiently and continuously engage desired sites on the inner wall of the blood vessel without causing damage to the blood vessel.